This invention is related to filters for digital data.
A data filter is a device that processes a stream of bits to achieve a desired transfer function. The commonest realization of a data filter is a shift register that is formed by cascading a plurality of elements that produce equal time delays. A resistor is connected to each such element at an output terminal. The other ends of all the resistors are tied together at a central point which is the output of the data filter. Well-known techniques of design allow a designer to select a desired number of stages in the shift register, the sign of the output at each stage of the shift register, and the magnitude of each of the resistors to achieve a desired transfer function. When so designed, such a filter is non-recursive. This means that its output is a function only of the input, and not of the previous output.
Data filters are of particular use for two applications in digital radio communications systems. One such use is a splatter filter for digital data. Another is to recover audio in a receiver for a continuously-variable-slope delta modulation (CVSD) system. A splatter filter is a low-pass filter that is so named because its purpose is to attenuate frequency components above an upper limit in a digital data stream, thus preventing the radio signal that is modulated with such data stream from "splattering" or spilling signal into adjacent channels. In the CVSD receiver, a data filter is useful for removing components at the bit rate that arise when recovering audio from a bit stream having CVSD modulation.
An analog filter could be used for either of these applications. However, a low-pass analog filter with a cutoff in the vicinity of 3 kHz requires element values that are difficult to obtain as a portion of an integrated circuit. In addition, if a circuit is otherwise adapted for realization in an integrated circuit, it is a relatively simple matter to increase the number of stages as desired to obtain a cutoff characteristic for a filter that is sharper than one easily obtainable with the discrete resistors, capacitors and inductors that are interconnected to form analog filters. It is particularly desirable in a splatter filter to have a sharp cutoff with a minimum amount of rolloff in the passband of the filter. This minimizes the delay or distortion of high-frequency information in the digital signal while enabling the designer to meet specifications for a maximum of allowable amount of adjacent-channel interference. Filters are used conventionally in CVSD systems in the conversion of the digital CVSD signal into audio. In the typical CVSD system a CVSD data stream is multiplied by an analog voltage to convert the data into a stream of bits that has pulse-amplitude modulation. This signal in turn is subjected to analog filtering to reconstruct the audio signal. This filter needs a low-pass characteristic in order to pass the reconstructed audio while removing both quantizing noise at the bit rate and aliasing noise that results from a spectral shift caused by the digitizing process. Aliasing places multiples of the fundamental audio spectrum periodically in frequency outside the passband. As with the splatter filter, the discrete resistors, inductors and capacitors that may be used to construct an analog filter for a CVSD receiver are difficult to achieve by the techniques used to obtain large-scale integrated circuits. With both the splatter filter and the CVSD filter, it would be desirable to have filters that could be realized as integrated circuits on semiconductor substrates.
In CVSD circuits that use analog low-pass filters to process the output of the CVSD multiplier, there is no effective way to combine multiplication and filtering and no reason for doing so. The application of a linear filter to a bit stream that has been subjected to pulse-amplitude modulation is a linear process that is cascaded with the CVSD modulator to form a CVSD receiver. If it were possible to combine filtering and multiplication, the result would be to minimize components and improve the effectiveness of the circuit.
It is an object of the present invention to provide an improved data filter.
It is a further object of the present invention to provide a multiplying data filter.
It is a further object of the present invention to provide a data splatter filter.
It is a further object of the present invention to provide a data splatter filter that is adapted for construction as an integrated circuit.
It is a further object of the present invention to provide a multiplying data filter that is adapted for construction as an integrated circuit.